In general, piezoelectric ceramics are ABO3 perovskite-type metal oxides, such as lead zirconate titanate (hereinafter referred to as “PZT”). However, PZT contains lead as an A site element, and its effect on the environment is regarded as a problem. Thus, there is a demand for piezoelectric ceramics of lead-free perovskite-type metal oxides.
NPL 1 discloses that, in a solid solution of a small amount of barium titanate in an antiferroelectric material sodium niobate, sodium niobate converts into a ferroelectric material. NPL 1 also discloses the remanent polarization, coercive field, piezoelectric constant, and electromechanical coupling coefficient of a compound having a barium titanate concentration in the range of 5% to 20% sintered at a temperature in the range of 1200° C. to 1280° C. The material described in NPL 1 is free of lead and potassium. Potassium is responsible for poor sinterability and low moisture resistance. The Curie temperature of the material described in NPL 1 is higher than the Curie temperature (110° C. to 120° C.) of a typical lead-free piezoelectric material barium titanate. NPL 1 discloses that the Curie temperature of the composition (Na0.9Ba0.1)(Nb0.9Ti0.1)O3 having the maximum piezoelectric constant d33=143 pC/N is 230° C.
PTL 1 discloses that the addition of cobalt to a piezoelectric ceramic that is a solid solution of sodium niobate and barium titanate (NN—BT-Co) improves the piezoelectric constant. It is also disclosed that a sample of the piezoelectric materials described in PTL 1 was difficult to polarize because of a low insulation resistance as low as approximately 106Ω.